This invention relates to television camera control and in particular to apparatus for improved image highlight discharge.
Television camera pickup tubes operate on the principle that light from an image strikes the photosensitive surface of the target of the tube. The target is a capacitor which is charged to cathode potential by the electron scanning beam. The light from the image discharges this capacitor and the scanning electron beam which scans the target in a predetermined pattern recharges the capacitor back to cathode potential. By measuring the current which flows to the target electrode during recharge by the electron beam and knowing the scanning pattern, it is possible to reproduce the tube image. The extent to which the target is recharged is determined by the current of the electron beam; the greater the beam current, the greater discharged potential that may be recharged. The electron beam current is determined by the tube control grid, or G1 voltage. Increasing G1 voltage increases beam current resulting in the ability to recharge a greater potential.
G1 voltage is normally set to allow the electron beam to recharge an image caused by a scene of normal brightness levels during a few scans of the electron beam. Because of the capacitor time constants involved, the electron beam will recharge the capacitors to approximately 70% of cathode potential in one scan. The subsequent scan will recharge 70% of the remaining potential difference. Therefore, the capacitors are recharged substantially to cathode potential in only a few scans. When scenes of high brightness levels are viewed, however, such as when the camera is faced toward a bright light or the sun, the target may be discharged to an extent that the potential of the electron beam is unable to recharge it fully, even with repeated scans. If the camera or the scene moves, the bright spots which were unable to be recharged will smear and move around, creating a comet-tail effect.
In order to eliminate this comet-tail effect, the G1 voltage and therefore the beam current must be increased to a level sufficient to recharge the target to cathode potential during the period of a few scan cycles. Operating the beam on a continual basis with enough current to discharge any highlights that may occur will eliminate comet-tailing but is undesirable for a number of reasons. One is that greater amounts of energy are consumed. Another is that increased beam current increases the size of the scanning spot, reducing the tube resolution. Still another reason for avoiding extended periods of elevated beam currents is to prolong tube life. High levels of beam current can appreciably shorten the life of the tube and can even cause tube damage. Further background information on camera tube operation and a discussion of highlight related problems can be found in U.S. Pat. No. 3,610,823.
A number of prior art methods have been used to increase beam current only in the presence of an image highlight with the remaining scanning done at the lower, normal beam current. The prior art methods, however, fail to eliminate many of the problems that occur in image highlight discharge. Among these is a method disclosed in U.S. Pat. No. 3,610,823 issued to Haenen. Haenen uses a continual increased beam current to one camera pickup tube. When the image output signal from that tube exceeds a threshold value, a control signal is generated which increases the beam current of the other tubes. This system, obviously, presents the problems of decreased resolution and shortened tube life of the tube having the increased beam current. Other prior art methods raise the beam current to a high level upon the sensing of an image highlight, without any regard for the highlight brightness level. This can result in circuit oscillation and decreased tube resolution.